In the production of highly pure metal powders, it is common practice to "bundle" liquid metal so as to form a relatively narrow stream, so that the metal may be subsequently atomized with an atomizing nozzle or a rotating disk atomizer. In lost-wax molding processes, it is common practice to pour pure molten metallic material into a mold with a stream former.
A known method of forming streams of molten material is so-called "drop-off melting", in which a cylindrical rod of metallic material is melted and supplied to a dispersion nozzle. One example of this method is disclosed in DE-A-3 433 458. In this example, the rod of material is pushed vertically against an induction coil. The coil has a longitudinal dimension less than the length of the rod, and defines a central aperture that is smaller than the diameter of the rod. The lower end of the rod is held with its front surface at an essentially constant axial distance above the induction coil. One disadvantage of this method is that the starting material must be provided in rod form.
In another known method for forming a stream of molten metallic material, molten material is poured from a ceramic crucible. The crucible can withstand the high temperatures of the molten material, and thus has the advantage that it does not need to be cooled. However, the crucible method is disadvantageous in that the molten material may be contaminated by contact with the ceramic crucible.
To avoid contamination of the material, pouring crucibles could be fabricated from metal. However, a metal crucible would have to be cooled, thus causing the molten material to tend to solidify when poured. If moderately large crucibles of metal were employed, with the material being melted using plasma or electron beam techniques, the aperture of the crucible from which the liquid metal stream flows would solidify more rapidly in inverse relation to the width of the aperture. The crucible technique is therefore unsuitable for use with powder generating devices, since known atomizing arrangements require relatively narrow streams of material.
It is therefore apparent that the need exists for a method and apparatus for forming streams of molten material that will generate a relatively narrow stream of molten material while avoiding the risk of solidifying, as well as selectively allowing the molten material to solidify and melt again.